1. Field of the Invention
This invention relates to liquid vapor delivery systems of the type employing bubblers and to methods of maintaining constant levels of reagent within bubblers.
2. Description of the State of Art
The semiconductor industry is very dependent upon sources of ultrahigh purity reagents. Other industries also have high purity requirements, but few compare with the purity requirements in the semiconductor industry. Liquid vapor delivery systems are used in a number of manufacturing processes. For example, liquid vapor delivery systems are used in the manufacture of optical wave-guides. Such systems are described in U.S. Pat. Nos. 3,826,560; 4,235,829; and 4,276,243, the disclosures of which are incorporated herein by reference. Thin films are sometimes produced by liquid vapor delivery system technology, see HANDBOOK OF THIN FILM TECHNOLOGY, Maissel and Glang, McGraw-Hill, New York, 1970. Film deposition techniques are generally described, as applicable to the semiconductor industry, in ENCYCLOPEDIA OF SEMICONDUCTOR TECHNOLOGY, Grayson (Ed) John Wiley, New York, 1984.
With particular application to the manufacture of semiconductor films and devices, it is known to provide semiconductor devices by reacting a silicon wafer, appropriately prepared with the semiconductor component pattern thereon, with the vapor from chemical liquid vapor source materials or dopants. Among these chemical vapor source materials are boron tribromide, phosphorous oxychloride, phosphorous tribromide, silicon tetrachloride, dichlorosilane, silicon tetrabromide, arsenic trichloride, arsenic tribromide, antimony pentachloride and various combinations of these. In the compound semiconductor industry, epitaxial III–V semiconductor films are commonly grown by metalorganic chemical vapor deposition (MOCVD) using liquid vapor source materials such as trimethylgallium, triethylgallium, trimethylaluminum, ethyldimethylindium, tertiary-butylarsine, tertiary-butylphosphine, and other liquid sources. Many II–VI compound semiconductor films are also fabricated using liquid sources.
A common technique used in vapor generating systems for delivering chemical vapor to a process chamber is to force a carrier gas to bubble through a chemical fluid in a bubbler and then deliver the resulting vapor from the bubbler to the process chamber. Traditional bubblers, including those utilized in presently available automatic refill systems, rely on relatively large fluid volumes to intrinsically compensate for deviations in fluid level which can negatively effect the resulting vapor concentration. Since vapor sources in the fiber optics and semiconductor industries are often hazardous fluids, there has been an increasing focus on the occupational safety and health concerns resulting from use of such fluids. This has resulted in reducing the maximum allowable volumes of many of these fluids within the work place. It is therefore desirable to reduce the required fluid volume at the point of vapor generation without compromising vapor concentration control.
Those familiar in the art recognize that absolute values of vapor delivered are not the prime importance, but the consistency of the vapor concentration is of utmost importance. Hence the value of source material delivered per unit of time is more important than source material delivered per unit of carrier gas. Therefore, temperature and carrier gas residence time (liquid level) become the prime variables in determining consistency.
Typically a bubbler container is comprised of a single vessel, which holds an expendable volume of vaporizable fluid. A carrier gas such as hydrogen, oxygen, argon, helium or nitrogen is introduced at the lower level of a fluid column, travels up through, and exits the fluid surface into a headspace. As the carrier gas passes through the fluid column it becomes entrained with vapor, which results in a corresponding reduction of the fluid volume. In some applications such decreases in the level of liquid within the bubbler would have little effect. In other applications, however, such as in vapor deposition procedures employed in constructing semiconductors and optical fiber preforms, significant variations in the level of liquid have a pronounced adverse effect. This is attributable to the fact that the rate of vaporization is not solely dependent upon the surface area of liquid within the bubbler which area can, of course, be maintained constant as by the use of cylindrically shaped vessels. The vaporization rate here however is also dependent upon several other factors including the flow characteristics of carrier gas bubbled through the liquid. For example, the size of the bubbles as they rise through the liquid has an effect on the rate of vaporization. The rate of flow of the carrier gas introduced into the bubbler itself has another effect on the rate of vaporization as also does the rise time of the bubbles, which, of course, depends on the depth at which they are introduced. The rate of heat transfer into the bubbler will also be affected by changes in the level of liquid. While theoretically possible to program a heat controller to account for these variables as changes in the level of liquid are continuously monitored, that approach is complex and fails to eliminate the need for some degree of level control to prevent complete depletion or flooding. Vaporization rate also defines the headspace present above and within the bubbler container, which has been found to negatively effect the vapor concentration and ultimate bubbler performance when not optimized.
Inasmuch as vapor extraction from a fluid volume results in depleting the fluid volume of a bubbler, causing variations in vapor concentration, a means of replenishing this fluid is desirable. Some methods include manually replacing the bubbler ampoule once the volume of fluid reaches a predetermined minimum acceptable level. Other manual methods rely on an auxiliary supply of fluid to replenish the bubbler during intermittent periods of non-use. These systems rely on weight scales, sight glasses, ultra sonic detection, infrared or mechanical float systems. All of which have serious drawbacks. Such methods can result in reducing many of the concerns associated with prior art expendable bubblers, such as reducing the risk of contamination during ampoule replacement or any necessary fluid replenishment, but create a host of potential problems including metal fatigue, sealing of dissimilar materials, and need for calibration procedures. With many of the advanced processes running short cycle times, the opportunity to refill the liquid level must be very fast and accurate, typically less than 20 cc's of source material.
In addition to manual replenishment of fluid, automatic bubbler refill systems are also available. However, such systems typically employ float coupled electronic devices, such as level controllers, to control the replenishment of fluid in the bubbler. Such devices are prone to failure, or inaccuracies and are generally the most common failure mechanism in the system. Other types of fluid level sensors such as optical, load cell monitoring of the contents, and resistance probes have been employed. However, the use of such devices can be costly, prone to error, and with many of the fluids being flammable, represent ignition sources if not properly rated and maintained. The corrosive and toxic nature of the materials commonly encountered greatly limit the materials and construction of sensors used in these demanding applications.
U.S. Pat. No. 4,235,829, which issued on Nov. 25, 1980 to Fred P. Partus is noted. In it, there is shown a liquid vapor delivery system, which comprises a deposition bubbler adapted to generate and to deliver vapor from a liquid contained therein and in a reservoir in fluid communication with the bubbler. Facilities are provided for sensing the level of the liquid contained within the bubbler and for providing gaseous head pressures in the reservoir of magnitudes dependent upon the sensed liquid level. The liquid level in the bubbler drops as liquid is vaporized and withdrawn from the bubbler whereupon the level is adjusted by increasing the pressure head in the reservoir to feed liquid to the bubbler. Although the system works well, perturbations in the deposition bubbler caused by a drop in liquid level and then a rise due to the changing of the pressure in the reservoir can to an extent affect adversely the rate of vaporization and hence the concentration level of the vapor. These level changes are increased as the rate of deposition and hence the rates of withdrawal of vapor are increased.
Additionally, commonly assigned U.S. Pat. No. 4,276,243, which issued on Jun. 30, 1981 to Fred P. Partus, discloses a similar liquid vapor delivery system, which utilizes the temperature of the liquid as the characteristic to be monitored and manipulated to control the concentration level at the desired value. However, while this system is effective, the slow rate at which the overall temperature of the liquid can be changed, particularly cooled, often results in an unwanted delay in achieving the desired concentration level corrections.
Accordingly, it is to the task of maintaining a substantial constant level of liquid within a bubbler while vapors from the liquid are being continuously or intermittently withdrawn to which the present invention is generally directed. More particularly, the invention is directed to level control systems and methods for use in vaporizing highly corrosive liquids which may easily become contaminated if brought into direct contact with materials of the type used in conventional level controllers such as floats and the like.